The desire to integrate data, voice, image and video over high speed digital trunks has led to the development of a variety of packet and cell switching techniques. One such technique is called Asynchronous Transfer Mode (ATM). ATM is a switching technology that provides users with the ability to connect to one or more users in a transparent fashion. Unlike the variable length packets used by frame relay services, ATM service is based on switching fixed length packets of data known as cells. Cell switching, as it is called, is gaining popularity for a variety of reasons. First, switch architectures can be optimized to switch cells at much higher speeds than variable length packets. Second, multiple services requiring a variety of quality of service guarantees can be provided simultaneously. ATM user traffic is first segmented into fixed length cells, transmitted, then reassembled back into its original form. This segmentation and reassembly (SAR) process is done in a standardized way, regardless of the carrier providing the ATM service.
Although the use of fixed length cells in ATM can be efficient in terms of allowing standardized switching apparatus to be used, for many applications the standard 53-byte cell provides too large a package for the data requiring transport through the network. As a result, much of the cell payload is merely “padding” and the transport of such padding wastes the available bandwidth of the ATM network. Several approaches to solve this problem present themselves.
For example, one could use a shorter length cell. As indicated, ATM uses a standard 53-byte cell, with 48-bytes of payload and 5-bytes of header information. Choosing a smaller cell size could result in less of the cell payload being filled by padding. However, shorter cells have two important disadvantages. First, such cells would be non-standard and, as a result, such cells could not be transported through ATM networks designed to accommodate only standard size cells. This lack of interoperability would likely mean that users would be disinclined to accept such a solution. Second, smaller cells would likely end up wasting more bandwidth than they would save because the ratio of header size to payload is much higher than for a 53-byte cell. Unfortunately, the header size could probably not be reduced from the current 5-byte size without a loss of functionality.
Another solution might be to use variable length packets, as is common in frame relay networks. This could conceivably avoid the need for padding altogether because packets could be “custom built” to the requirements of the user data. Unfortunately, the very fact that such varying packet sizes are allowed within frame relay networks means that the switches used to transport the packets across the network must be more complex than their ATM counterparts. As a result, such switches are generally slower than ATM switches. Further, in networks where variable length packets are used, it is difficult to make real time service guarantees without the use of complex servicing and queuing algorithms and some limitations on packet size.
Yet another solution might be to pack multiple payloads into one cell at a source and then pull these payloads apart at a destination. The ATM Forum has begun discussions regarding such bundling of data channels within a single cell, however, it is recognized that this will only provide a solution for channels which have the same source and destination nodes. For example, referring to FIG. 1, multiple calls from end-system A to end-system B could be bundled together, but such calls could not be bundled with calls from end-system C to end-system D, even though they share a network trunk between switches 5 and 10. This results in less than ideal efficiency and also requires higher layer signaling support to assign slots in each cell to various channels and to manage the slot allocation across a network. Possibly, switches 5 and 10 could provide a server functionality which terminates all virtual channels (VCs) and swaps around sub-channels to improve efficiency on the network link between the switches. However, this solution would introduce significant complexity in switches 5 and 10 because the switches would now be required to understand the end-to-end channel assignments and pack and unpack data from many cells simultaneously. Besides the increased hardware complexity required for this function, the signaling and management of the reassignment of sub-channels would require complicated signaling.
Accordingly, what is needed is a scheme for ensuring that as much available payload space within the fixed length cells of ATM networks are used to transport user data rather than padding without resulting in the drawbacks of the methods described above.